1. Field of the Invention
This invention relates to an optical information processing apparatus, such as an optical disc apparatus, in which a recording medium on which tracks are concentrically or spirally formed is rotated the tracks are scanned by a light beam to thereby accomplish recording and/or reproduction of information.
2. Related Background Art
Heretofore, an optical disc apparatus has assumed a construction such as shown in FIG. 1 of the accompanying drawings. Here, information tracks are concentrically or spirally arranged on a disc 1 which is an information carrier. This disc 1 is rotated by a motor 2, and a light beam 4 is applied from an optical head 3 to the disc 1 to thereby accomplish recording or reproduction of information. The optical head 3 is supported for movement in the radial direction of the disc 1 by a guide rail 10, and is driven by a linear motor comprising a coil 8a and a magnet 8b.
The optical head 3 is constructed as shown, for example, in FIG. 2 of the accompanying drawings. A semiconductor laser 16 is provided in the optical head 3. A light beam emitted from the semiconductor laser 16 passes through a collimator lens 17 and a beam splitter 18 and is condensed on the optical disc 1 by an objective lens 5. The light reflected by the optical disc 1 again passes through the objective lens 5 and is reflected by the beam splitter 18. This reflected light is condensed by a sensor lens 19 comprising a rotation-symmetrical lens and a cylindrical lens, and is received by a four-division detector 20. The four-division detector 20 detects a tracking error signal and a focusing error signal by a method as described in U.S. Pat. No. 4,410,969. The detected tracking error signal is put to a tracking actuator 22 through a tracking control circuit 21. The tracking actuator moves the objective lens 5 in a direction perpendicular to the optical axis and crossing the tracks (the racking direction) in accordance with the signal, and controls the light beam so as to accurately trace the tracks. On the other hand, the objective lens is moved in the direction of the optical axis by a mechanism, not shown, in accordance with the focusing error signal, whereby auto-focusing is accomplished.
However, there is a limit to the range of movement of the objective lens by the tracking actuator. Also, the center of the objective lens greatly deviates relative to the center of the light beam from the semiconductor laser, the imaging performance is deteriorated. Therefore, in U.S. Pat. No. 4,627,039, as shown in FIG. 3 of the accompanying drawings, the position of the objective lens in the tracking direction relative to the base 3a of the optical head is detected by a lens position sensor 6. The detected lens positional signal is input to the linear motor coil 8a through a compensatory controller 7 shown in FIG. 1 which comprises a control stabilizer, a driving amplifier, etc. By such a construction, the optical head 3 follows the movement of the objective lens 5 and moves in the direction across the tracks, whereby the great deviation of the objective lens as mentioned above is prevented. For example, when tracks are spirally formed on the disc 1, the objective lens 5 gradually moves in one direction while tracing the tracks. Following this movement of the objective lens 5, the optical head 3 also moves in the same direction.
However, in the apparatus of the above-mentioned U.S. Pat. No. 4,627,039, the optical head 3 also follows the movement of the objective lens attributable to the eccentricity of the disc 1. This eccentric component is within the tolerance of the tracking servo and does not affect the deterioration or the like of the imaging performance. Accordingly, it is originally unnecessary that the optical head follow such an eccentric component. Thus, in the apparatus according to the prior art, power consumption has been great, because the optical head effects wasteful movement. Also, depending on the control band set, movement of the optical head has resulted in increased eccentric disturbance to tracking servo.